Solid propellant rocket motors or gas generators of the end or "cigarette" burning type wherein only the exposed face of the propellant burns are commonly used in the aerospace field of activity and perform very satisfactorily for many different propulsion applications. They are useful where a substantially constant burning rate of the propellant is not objectionable and can be tolerated. In some instances, however, it is important that the burning rate of the propellant be controlled to some extent over the rate of normal end burning, such as in the event of a motor for a programmed missile where the amount of thrust developed by the rocket motor may need to be increased or decreased back to normal burning thrust in order for the missile to follow its intended path of flight.
One way which has been proposed to achieve such a purpose is shown in the United States patent to Glick et al U.S. Pat. No. 3,381,476 wherein filaments or wires are embedded in the propellant of an end burning rocket motor during its construction and are adapted to be pulled away, during motor operation, from the burning face at a rate greater than the rate of burning of the propellant, thus producing a hole where the filament has vacated and thus exposing more propellant to burning than was previously available at the end face. This exposed propellant then burns in addition to that which is burning at the propellant end face, the total burning rate is increased and more combustion gases are produced resulting in more thrust or mass flow output by the rocket motor. Conversely, if the filament withdrawal rate is slowed or stopped, the filament uncovers less propellant surface in the filament hole as the end burning face "catches up" and a slower overall burning rate is achieved with less total thrust or mass flow output as a result. In other words, the thrust or mass flow produced by the end burning face of the propellant remains the same until the propellant is consumed but the amount of additional thrust or mass flow produced by burning along the filament holes up to the filament ends is controlled. In this way, a useful measure of control is achieved over the total burning rate and thrust or mass flow of the motor.
Another patent which describes filament withdrawal in an end burning solid propellant rocket motor is U.S. Pat. No. 3,630,028 to Leonard Caveny in which he not only includes filaments but also includes cutters attached to them which physically slice into the propellant in order to expose more propellant to burning in order to increase the overall burning rate.
In filament withdrawal rocket motors of the types described above, an inherent problem arises, however, in that the clearance between the filaments and the propellant needs to be very small in order to prevent burning, or what has been termed "flashback", along the wire as unwanted burning occurs due to the exposure of propellant back along the wire. On the other hand, if the clearance is made purposely very small in order to prevent flashback, any slight distortion of the propellant during motor operation is likely to cause binding or sticking of the filament in its hole and thus to interfere with proper operation of the device with resultant lack of control over the burning rate and motor thrust or mass flow output.
The present invention solves this problem by providing a separator between the propellant grain and its casing which serves a twofold purpose. In the first of these, channels are provided essentially between the casing and grain which are open to the combustion chamber and thus allowing the external pressure upon it to become substantially equal to that existing in the combustion chamber so that pressure induced distortion of the grain cannot occur to a degree which will produce either the flashback effect or binding of the filament. Secondly, the separator is made sufficiently resilient that the grain is not subject to mechanical distortions which could otherwise be of a magnitude to cause distortion of the filament holes and cause the same flashback and binding problems. Because of this structure, the filaments can be fitted closely enough to prevent flashback but will not bind because the distortion of the propellant no longer occurs to a significant degree and the filaments and their surrounding holes remain substantially straight, coaxial and unenlarged.
The U.S. Pat. No. 3,108,433 to M. G. DeFries et al is mentioned as showing a gap between the propellant grain and housing but it includes inhibiting or insulating wrappings which in themselves confine the propellant grain and may produce propellant grain distortions. This patent does not show filaments in the grain.